1. Field of the Invention
This invention relates to a magneto-optical recording medium having a large magneto-optical effect when exposed to short-wavelength light and hence is suitable for high-density recording.
2. Description of the Prior Art
Magneto-optical recording has been put to practical use as an optical recording method that permits rewriting. Heretofore, single-layered rare earth-transition metal amorphous films having a strong magnetic anisotropy in a direction perpendicular to the film surface have been employed as magneto-optical recording films.
In particular, TbFeCo alloy amorphous films are being researched and developed for practical use. An increase in the recording density is a major aim in the field of magneto-optical recording. Shortening of the wavelength of writing/reading light is currently being studied as a method of achieving this. To increase the magneto-optical recording density in the future, it is indispensable to develop magneto-optical materials that have a high magneto-optical effect (the Kerr effect and Faraday effect) at short wavelengths. With such conventional TbFeCo alloy amorphous films, the magneto-optical effect tends to decrease monotonically as the wavelength of a laser beam becomes shorter, and sufficiently large Kerr rotation or Faraday rotation is not obtained at short wavelengths, which results in a drastic decrease in the output when it is read out by a laser beam.
On the other hand, there is a known amorphous film that is an alloy of a transition metal consisting mainly of Co and Fe and of rare earth elements consisting mainly of Nd and Pr (see, for instance, T. R. McGuire et al., "Magneto-optical Properties of Nd-Co and Nd-Fe Alloys," J. Appl. Phys. 61(8), Apr., 15, 1987, pp. 3352-3354). This film, although it has a large magneto-optical effect at short wavelengths, has in-plane magnetization but not perpendicular magnetization, and cannot realize high-density recording.
A double-layered film formed by laminating a Nd alloy rare earth-transition metal amorphous film and a TbFeCo amorphous film to utilize exchange coupling between them is proposed by Ito, et al. in "Magnetic and Magneto-optical properties of Nd alloy multilayers," Digest of the 13th annual conference on magnetics in Japan (1989), p. 325. However, the double-layered film disclosed therein has a Kerr rotation angle of 0.3 degrees at a wavelength of 400 nm, and does not fully realize intrinsic Kerr rotation of Nd alloys.
Among materials other than amorphous materials, crystalline Co and Fe are known to have large magneto-optical effects at short wavelengths. However, they are not available in the form of perpendicularly magnetized films and have received almost no attention heretofore as magneto-optical materials.
Here again, attempts have been made to obtain a perpendicular-magnetized film by laminating an in-plane magnetized film Co and a perpendicular-magnetized film TbFeCo to utilize exchange coupling between two films (see, H. Wakabayashi, et al., "Magnetic and magneto-optical properties of Co/TbFeCo exchange coupled films," Digest of the 13th annual conference on magnetics in Japan (1989), p. 326). However, in order to obtain a double-layered film that has good squareness according to the proposed approach, the thickness of Co must be 25 .ANG. or less. Therefore, this film can have only a low Kerr rotation angle derived from the TbFeCo film alone at short wavelengths.